In receivers and transmitters, complex signals are often represented as two real signals denoted in-phase (I) and quadrature-phase (Q). The complex signal is then given as S=I+jQ where j=i√−1. Referring to FIG. 1, the typical well-known quadrature receiver architecture is shown wherein RF signal 100 is down converted to I/Q signals 101, which can a baseband frequency or an IF frequency (such as in so-called “low-IF” receivers).
In FIG. 1, down conversion is illustrated for the case where the down conversion is to baseband. Spectrum of RF signal 102, the spectrum of local oscillator (LO) tone 103, and the resulting down converted spectrum 104 of S=I+jQ is shown. The desired result is a frequency translation of the signal spectrum but as spectrum 104 shows, there will also be an undesired image signal 105, shown hatched. This is a mirror-imaged (spectrally inverted) and attenuated version of the desired signal. The image is cause by amplitude and phase imbalances between the I and Q mixing paths. Usually the phase difference is caused by imperfections in the 90-degree phase split for the I and Q LO signals.
Up conversion from baseband I and Q signals to an IF or RF signal can also create an undesired image even when the LO signals driving the mixers are in perfect quadrature and amplitude balance. If the I and Q signals are not amplitude and phase balanced, a spectrally inverted image is present in the IF/RF and overlays the desired IF/RF signal.
It is desirable to reduce the image as much as possible and to this end it may be necessary calibrate the I and Q signals so as to suppress amplitude and phase errors. The calibration mechanism should preferably be continuous so that it can track changing amplitude and phase errors during without interrupting reception. The errors tend to drift during reception due to factors such as temperature, receive signal level, etc.
The conceptually most obvious way of adjusting the amplitude error is simply to insert a variable gain amplifier in either the I or Q path or both. Similarly, the phase error can be corrected by introducing a variable phase adjustment in series with the 90 degree LO splitter. This leaves the challenge of devising a method for measuring amplitude and phase differences between I and Q accurately and using that information to drive the correction circuitry. The implementation of phase correction and, in particular, accurate phase measurement is very challenging, especially under the constraint of continuous calibration because the properties of the received signal are in most cases unknown and therefore many well-known techniques such as zero-crossing detection are not applicable.
Lui, U.S. Pat. No. 6,560,449, entitled “Image rejection I/Q demodulators”, issued May 6, 2003 discloses a feedback technique for reducing the image response of a receiver. The image/signal ratio is measured on the demodulator I/Q mixer outputs by detecting the phase and amplitude differences of the I and Q demodulated signals, then amplitude and phase control is applied to the quadrature LO generator driving the I and Q mixers to reduce the image response. The image/signal detector is calibrated during interstitial intervals between data packets. This approach requires interruption of the primary signal demodulation for calibration of the detector and a phase and amplitude adjustable LO generator.
For these reasons, it would be desirable if the I/Q phase and amplitude balance could be corrected without needing phase measurement or correction. Furthermore, it would be advantageous if this technique could be implemented entirely at the I/Q signals without requiring any intervention into the sensitive RF circuits.